In an uplink in mobile communications, when a terminal, which is even not transmitting upstream data, receives downstream data, it transmits a positive response/negative response (Acknowledgement/Negative Acknowledgement, which will be designated as “ACK/NACK” hereinbelow) indicating whether information on a downlink was successfully received without error, or information indicating the quality of a channel (Channel Quality Indicator, which will be designated as “CQI” hereinbelow), which indicates the quality of communication in the downlink, using an uplink control channel.
Currently, in LTE (Long Term Evolution) for which standardization is being developed by 3GPP (3rd Generation Partnership Project), the format of a control signal is different in transmitting an ACK/NACK and/or a CQI using an uplink control signal (Physical Uplink Control Channel, which will be designated as PUCCH hereinbelow) between a case in which only an ACK/NACK is transmitted, a case in which only a CQI is transmitted, and both an ACK/NACK and a CQI are transmitted.
FIG. 1 shows examples of (a) a format in transmitting only an ACK/NACK, (b) a format in transmitting only a CQI, and (c) a format in transmitting a CQI and an ACK/NACK. One slot represents 0.5 ms and is composed of seven long blocks (LB's), and one transmission time interval (TTI: Transmission Time Interval) is composed of two slots. The TTI refers to a period of time over a plurality of blocks that are transmitted between a physical layer and a MAC layer at a time. As can be seen from FIG. 1, when a CQI and an ACK/NACK are transmitted at the same time, the number and position of long blocks allocated to the CQI and/or ACK/NACK are different as compared with a case in which only an ACK/NACK is transmitted as control information or a case in which only a CQI is transmitted as control information.
FIG. 2 shows an example of a slot configuration in LTE. A PUCCH is multiplexed on both sides of a system bandwidth. Although the PUCCH portion includes a reference signal (RS) for demodulating the PUCCH as shown in FIG. 1, it is omitted from the illustration in FIG. 2.
The PUCCH and reference signal for demodulating it employs a CAZAC (Constant Amplitude Zero Auto-Correlation) sequence. The CAZAC sequence is characterized in having a constant amplitude and an autocorrelation value of zero at a non-zero phase difference in the time domain and frequency domain. An example of the CAZAC sequence is a Zadoff-Chu sequence given as follows (see Non-patent Document 1):
                                          c            k                    ⁡                      (            n            )                          =                  {                                                                                                                exp                      ⁡                                              [                                                                                                            j                              ⁢                                                                                                                          ⁢                              2                              ⁢                                                                                                                          ⁢                              π                              ⁢                                                                                                                          ⁢                              k                                                        L                                                    ⁢                                                      (                                                                                                                            n                                  2                                                                2                                                            +                              n                                                        )                                                                          ]                                                                                                                                                                                                                                    in                              ⁢                                                                                                                          ⁢                              case                              ⁢                                                                                                                          ⁢                              that                              ⁢                                                                                                                          ⁢                              a                              ⁢                                                                                                                          ⁢                              sequence                              ⁢                                                                                                                          ⁢                              length                                                                                                                                                                                          L        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                                                              exp                      ⁡                                              [                                                                                                            j                              ⁢                                                                                                                          ⁢                              2                              ⁢                                                                                                                          ⁢                              π                              ⁢                                                                                                                          ⁢                              k                                                        L                                                    ⁢                                                      (                                                                                          n                                ⁢                                                                                                      n                                    +                                    1                                                                    2                                                                                            +                              n                                                        )                                                                          ]                                                                                                                                                                                                                                    in                              ⁢                                                                                                                          ⁢                              case                              ⁢                                                                                                                          ⁢                              that                              ⁢                                                                                                                          ⁢                              a                              ⁢                                                                                                                          ⁢                              sequence                                                                                                                                                                                          length                              ⁢                                                                                                                          ⁢                              L                              ⁢                                                                                                                          ⁢                              is                              ⁢                                                                                                                          ⁢                              an                              ⁢                                                                                                                          ⁢                              odd                              ⁢                                                                                                                          ⁢                              number                                                                                                                          ⁢                                                                                                                                                      ⁢                                                          ⁢              n              ⁢                              :                            ⁢                                                          ⁢              0                        ,            1            ,            …            ⁢                                                  ,                                                  ⁢                          L              -                              1                ⁢                                                                  ⁢                k                ⁢                                  :                                ⁢                                                                  ⁢                an                ⁢                                                                  ⁢                index                ⁢                                                                  ⁢                of                ⁢                                                                  ⁢                a                ⁢                                                                  ⁢                sequence                ⁢                                                                  ⁢                                  (                                      k                    ⁢                                                                                  ⁢                    and                    ⁢                                                                                  ⁢                    L                    ⁢                                                                                  ⁢                    are                    ⁢                                                                                  ⁢                    integers                    ⁢                                                                                  ⁢                    that                    ⁢                                                                                  ⁢                    are                    ⁢                                                                                  ⁢                                          coprime                      .                                                        )                                                                                        [                  EQ          .                                          ⁢          1                ]            
User multiplexing methods for the PUCCH include Frequency Division Multiplexing (FDM) and Code Division Multiplexing (CDM) (see Non-patent Document 2). In CDM, each user uses the same CAZAC sequence but with a cyclic shift specific to the user (see Non-patent Document 3). This ensures orthogonality between users. FIG. 3 is a diagram for explaining the cyclic shift. Representing a unit amount of the cyclic shift as ΔT ((long block length)/6 in the drawing), a cyclically shifted sequence i (i=1, 2, 3, 4, 5, 6) is created by shifting a basic sequence with a tail of ΔT×(i−1) brought to the head. To keep inter-user orthogonality, ΔT should be larger than a latest path in a propagation channel.
Since the ACK/NACK has a small amount of information to be transmitted (basically, one bit), it is possible to further apply block spread along a time axis to thereby increase the number of multiplexed users (see Non-patent Document 4). FIG. 4 is a diagram showing an example of block spread. Since in the drawing, three long blocks are used for a reference signal and four long blocks are used for an ACK/NACK, block spread is applied to the reference signal with an user-specific orthogonalizing code having a code length of three and to the ACK/NACK with the code having a code length of four. The number of multiplexable users is equal to the code length, and in this case, it is three at maximum from the length of the sequence for the reference signal having a shorter code length. Therefore, assuming that the number of multiplexable users is six using the cyclic shift, eighteen users, three times the six users, can be multiplexed within the same frequency.
For a user to whom a resource for downstream data is to be allocated using a downlink L1/L2 control signal, a PUCCH resource for transmitting an ACK/NACK is associated one to one with the index of a control channel for use in scheduling, and no signaling is performed by agreement.
FIG. 5 shows an example of the format of a downlink frame. First two OFDM symbols represent L1/L2 control signals, which are composed of Downlink grants (each designated as DL grant in the drawing) containing information on allocation of a downlink shared channel, and Uplink grants (each designated as UL grant in the drawing) containing information on allocation of an uplink shared channel. The Downlink grants #0-#N in FIG. 5 contain information on resources allocated to DL Data #0-#N and identifiers of users to receive the data, respectively.
FIG. 6 shows an example of association between Downlink grants and PUCCH resources. It should be noted that while PUCCH resources are currently given no indices, the following description will be made as if they were given indices.
A resource for transmitting a Downlink grant is composed of CCE's (Control Channel Elements), and each CCE constitutes a unit resource. The index of a CCE is associated one to one with the index of a PUCCH resource. While a Downlink grant uses at least one CCE or more, a PUCCH resource associated with the index of one of the CCE's used for one Downlink grant is a resource allocated to a user for that Downlink grant.
FIG. 6 shows a case in which a resource for transmitting a Downlink grant is composed of one CCE, and each CCE is associated one to one with a PUCCH resource. Therefore, a user for a Downlink grant #0 uses a PUCCH resource associated with a CCE (index #0) of the Downlink grant #0 to transmit an ACK/NACK for data allocated by the Downlink grant #0.
This eliminates the need for use of signaling in allocation of PUCCH resources, and it is possible to reduce the signaling overhead.
Non-patent Document 1: B. M. Popovic, “Generalized Chirp-Like Polyphase Sequences with Optimum Correlation Properties,” IEEE Transactions on information Theory, Vol. 38, No. 4, pp. 1406-1409, July 1992.
Non-patent Document 2: 3GPP R1-063448, Qualcomm, “Structure and Link Analysis UL Control Signaling,” November 2006.
Non-patent Document 3: 3GPP R1-060925, Texas Instruments, “Comparison of Proposed Uplink Pilot Structures For SC-OFDMA,” March 2006.
Non-patent Document 4: 3GPP R1-071293, Qualcomm Europe, “Link Analysis and Multiplexing Capability for UL ACK,” March 2007.